<!DOCTYPE html>
<html lang="zh">

<head>
  <meta charset="UTF-8" />
  <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>html5 canvas炫酷旋转银河系星空背景特效</title>
  <style type="text/css">
    body {
      background: #060e1b;
      overflow: hidden;
    }
  </style>
</head>

<body>
  <div class="htmleaf-container">
    <canvas id="canvas"></canvas>
  </div>
  <script type="text/javascript">
    "use strict";
    var canvas = document.getElementById("canvas"),
      ctx = canvas.getContext("2d"),
      w = (canvas.width = window.innerWidth),
      h = (canvas.height = window.innerHeight),
      hue = 217,
      stars = [],
      count = 0,
      maxStars = 1400;

    // Thanks @jackrugile for the performance tip! http://codepen.io/jackrugile/pen/BjBGoM
    // Cache gradient
    var canvas2 = document.createElement("canvas"),
      ctx2 = canvas2.getContext("2d");
    canvas2.width = 100;
    canvas2.height = 100;
    var half = canvas2.width / 2,
      gradient2 = ctx2.createRadialGradient(half, half, 0, half, half, half);
    gradient2.addColorStop(0.025, "#fff");
    gradient2.addColorStop(0.1, "hsl(" + hue + ", 61%, 33%)");
    gradient2.addColorStop(0.25, "hsl(" + hue + ", 64%, 6%)");
    gradient2.addColorStop(1, "transparent");

    ctx2.fillStyle = gradient2;
    ctx2.beginPath();
    ctx2.arc(half, half, half, 0, Math.PI * 2);
    ctx2.fill();

    // End cache

    function random(min, max) {
      if (arguments.length < 2) {
        max = min;
        min = 0;
      }

      if (min > max) {
        var hold = max;
        max = min;
        min = hold;
      }

      return Math.floor(Math.random() * (max - min + 1)) + min;
    }

    var Star = function () {
      this.orbitRadius = random(w / 2 - 50);
      this.radius = random(100, this.orbitRadius) / 10;
      this.orbitX = w / 2;
      this.orbitY = h / 2;
      this.timePassed = random(0, maxStars);
      this.speed = random(this.orbitRadius) / 100000;
      this.alpha = random(2, 10) / 10;

      count++;
      stars[count] = this;
    };

    Star.prototype.draw = function () {
      var x = Math.sin(this.timePassed + 1) * this.orbitRadius + this.orbitX,
        y = (Math.cos(this.timePassed) * this.orbitRadius) / 2 + this.orbitY,
        twinkle = random(10);

      if (twinkle === 1 && this.alpha > 0) {
        this.alpha -= 0.05;
      } else if (twinkle === 2 && this.alpha < 1) {
        this.alpha += 0.05;
      }

      ctx.globalAlpha = this.alpha;
      ctx.drawImage(canvas2, x - this.radius / 2, y - this.radius / 2, this.radius, this.radius);
      this.timePassed += this.speed;
    };

    for (var i = 0; i < maxStars; i++) {
      new Star();
    }

    function animation() {
      ctx.globalCompositeOperation = "source-over";
      ctx.globalAlpha = 0.8;
      ctx.fillStyle = "hsla(" + hue + ", 64%, 6%, 1)";
      ctx.fillRect(0, 0, w, h);

      ctx.globalCompositeOperation = "lighter";
      for (var i = 1, l = stars.length; i < l; i++) {
        stars[i].draw();
      }

      window.requestAnimationFrame(animation);
    }
    animation();
  </script>
</body>

</html>